1. Field of the Invention
The present invention relates to a liquid crystal display device having an improved viewing angle characteristic.
2. Description of the Related Art
In a liquid crystal display device, it is well known that the contrast of the display in the case of taking an oblique view of the image area is different from the contrast of the display in the case of taking a front view of the image area (the viewing angle characteristic). Therefore, there is a demand for a liquid crystal display device having an improved viewing angle characteristic.
Japanese Unexamined Patent Publications No. 1-270024 and No. 2000-29010 disclose a liquid crystal display device including a vertical alignment type liquid crystal cell, first and second polarizers arranged on either side of the liquid crystal cell, a first λ/4 plate arranged between the liquid crystal cell and the first polarizer, and a second λ/4 plate arranged between the liquid crystal cell and the second polarizer. When the polarizers and the λ/4 plates are arranged as described above, it is possible to improve the viewing angle characteristic in the case of taking an oblique viewing of the image area.
However, although the viewing angle characteristic of the liquid crystal display device having the polarizers and λ/4 plates can be improved, the range of the viewing angle, in which a viewer can see the display with high contrast, is relatively small.
Also, as a technique for improving the viewing angle characteristic of the liquid crystal display device, there is provided a technique of alignment division. In the technique of alignment division, one pixel is divided into a plurality of regions or domains, the states of alignment of which are different from each other, so that a viewer can see a display with high contrast even when the viewer takes an oblique view of the image area in the same manner as that when the viewer takes a front view of the image area. Especially, the assignee of the present application proposes a vertical alignment type liquid crystal display device having structures or slits which linearly extend on or in electrodes on the substrates between which the liquid crystal layer is interposed.
In this liquid crystal display device, most of the liquid crystal molecules are aligned substantially perpendicularly to the substrate surfaces when a voltage is not applied. However, liquid crystal molecules located close to the structure or slit tend to be aligned perpendicular to the wall surface of the structure or slit and pre-tilted with respect to the substrate surface. When voltage is applied, liquid crystal molecules located close to the structure or slit are inclined according to the pre-tilt so that they are inclined to a predetermined direction. Therefore, most of the liquid crystal molecules are inclined according to the liquid crystal molecules located close to the structure or slit.
The direction of alignment of liquid crystal molecules located on one side of the structure or slit is opposite to the direction of alignment of liquid crystal molecules located on the other side of the structure or slit. Therefore, two regions, the directions of alignment of which are different from each other, are formed on either side of the structure or slit. Therefore, even if rubbing is not conducted on the liquid crystal display device, it is possible to realize the alignment division similar to that of the liquid crystal display device in which pre-tilt is provided by rubbing. Therefore, when alignment division is conducted as described above, it is possible to obtain a viewing angle characteristic in which the viewing angle range is wide and the contrast is high. The liquid crystal display device having alignment division is disclosed, for example, in Japanese Unexamined Patent Publication No. 11-352489.
Japanese Patent No. 2945143 discloses a liquid crystal display device in which a polymer dispersed type liquid crystal display device is interposed between polarizers in a Cross Nicol arrangement. Japanese Unexamined Patent Publication No. 2000-347174 discloses a network-like polymer dispersed type liquid crystal display device.
In the above described liquid crystal display device having alignment division, the state of alignment of most liquid crystal molecules in one pixel is approximately controlled according to the predetermined structures or slits when voltage is applied. However, sometimes, the state of alignment of a portion of the liquid crystal molecules in one pixel cannot be controlled according to the predetermined structures or slits. For example, liquid crystal molecules located close to the bus lines located in the periphery of the pixel tend to be aligned perpendicularly to the wall surface of the bus lines. Therefore, the state of alignment of the liquid crystal molecules is different from the state of alignment of the liquid crystal molecules controlled according to the predetermined structures or slits, which could be a cause of deterioration of brightness. Liquid crystal molecules on the predetermined structures or slits are aligned in parallel to the predetermined structures or slits. The polarizers are arranged so that the axes of polarization can form an angle of 45° with respect to the director of the liquid crystal molecules when voltage is applied. However, a portion of the liquid crystal molecules become parallel to the axis of polarization, which could be a cause of deterioration of brightness.